Aesthetics and function

Here's a pretty effective graphic that shows the rising trend in gas/petrol prices since January, and in particular, the far bigger jump in California against the national average.

It's interesting to examine the array of aesthetic "enhancements" that was deployed:

Typically, modest coloring aids comprehension but less is always more. For this data set, two colors suffice, one for California (and its metro areas) and one for the national average

It is always preferable to place data labels next to the data, rather than deploying a maze of lines leading to other parts of the chart, as is the case here. The legend would have been clearer if placed uniformly on the right hand side of the chart, next to the end of the lines

The axis labels can be vastly improved. On the price (vertical) axis, the only relevant numbers are the average prices for each region. So I'd remove the vertical axis completely, put the region labels on the right side as suggested above, and put the regional average prices next to the labels

On the time (horizontal) axis, the choice of 7-day intervals is somewhat baffling. If weekly prices are of concern then it'd have been better to label them as "week 1", "week 2", etc., rather than 6, 13, 20, 27... In this case, nothing is lost by marking only the beginning and middle of each month

Grid-lines should be removed unless this is a chart intended to be used as graph paper, as in science lab

The practice of shading the plot area should be abolished: the labelling of month on the time axis is sufficient, anything else is redundant

Finally, if the intent of the chart was to judge the difference between California and national gas prices, then a much longer time horizon is required. Our curiosity is piqued by the convergence of the lines at the start of 2006; what had been the trend the year before?

Thanks Robert for the data source. I think it confirms that they were plotting weekly gas prices; the point that using generic "week 1", "week 2" etc. labels would have improved readability is still valid, I believe.

Those are prices on the reported day, not average prices for a week. From the EIA: "Every Monday, retail prices for all three grades of gasoline are collected by telephone from a sample of approximately 900 retail gasoline outlets. [...] The reported price includes all taxes and is the pump price paid by a consumer as of 8:00 A.M. Monday."

I think the use of dates is reasonable, and likely to be of more use than 'week 1' etc.

The main problem is, I think, the fact that the caption is making a comparison to a year previously, but the graph covers only 3 months. In fact, the caption suggests that there had been a drop in prices prior to January, but the graph hides this.